1. Field
The invention relates to video tape recorder/reproducer speed control circuits, and particularly to a speed measuring circuit for identifying the speed at which the tape was previously recorded.
2. Prior Art
Tape speed on playback is normally set by manual means on video tape recorder/reproducers (VTR) since in the past most television stations operated at the standard tape speed of 15 inches/second (ips). However, since video tape has become more expensive, and has been improved to allow broadcast quality recording at video bandwidths with slower tape speeds, a larger number of broadcasters are employing the 71/2 ips standard. As a result, a mixture of tapes recorded at both 71/2 and 15 ips are being used, and an automatic switching method is desirable to preclude operator error during playback of the tapes.
The pre-recorded control track on a video tape is an expeditious means for sensing the speed used for recording the tape. However, on quadruplex VTRs the standard control track is a composite signal of multiple frequencies and includes a 240 Hz signal with a large component of third harmonic distortion, frame pulses, etc. Thus it is difficult to extract the 240 Hz signal recorded in the composite control track with any degree of accuracy, over the range of tape speeds encountered when rapidly moving tape.